Depends on your friend. Are they somone you trust? Or more of an aquaintance? If they are somone you trust, you could “feel them out” and see if they actually do think you are a vampire. If they do, talk to them about it. Clear up any misconceptions right off the bat, so to speak. If they don't seem to know after all, then don't worry.
Some people will also play it off if they don't want another to really know. Acting the part, over doing the vampire schtick, and generally not being overly serious about it can make others think “oh, they are just goofing around, not really thinking they are a vampire”. and blow it off that way.
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